Coleco ADAM Digital Data Pack
While these look like normal cassettes, they are not directly interchangeable; a hole on the case is in a different position than on a standard audio cassette, which prevents audio cassettes from being inserted into Coleco Adam tape drives or Adam data packs being inserted in audio cassette players, though "hardware hackers" have succeeded in making such uses by drilling appropriate holes. This doesn't always work, however; it takes a very high-quality cassette to work reliably under the conditions of the Adam drive, which stores data at a high density and advances and rewinds the tape at high speeds to do random access of data blocks. Also, the tapes must be specially formatted before being used; new Adam tapes came this way, but any other cassette being prepared for use on an Adam needs to be formatted first on a machine designed for it, since the Adam itself wasn't able to format a tape.
The tape drive uses two tracks of the tape, and has two basic formats for storing data: GW and HE. Each of them stores a total of 256 kilobytes in 1-kilobyte blocks, using 128 blocks per track.
The GW format stores blocks 0 through 7F (hex) in order on the first track (track 0), and similarly 80 through FF on the second track (track 1).
The HE format stores 40 through 7F, then blocks 0 through 3F, on track 0, and 80 through FF in order on track 1.
GW-format tapes are used to store data sequentially without any directory structure or filesystem, and the Adam's operating system offers no ability to copy this format.
HE-format tapes can have an EOS filesystem (the native OS of the Adam) with a directory, and are treated similarly to disks by the Adam.